feat(floating-ai): step 5 — add ai:action IPC side-channel

Add a new ai:action IPC channel so the renderer can react to AI tool
side-effects (task creation, checkpoint/task suggestions). Also mark
AI-created tasks with isAiSuggested: 1 in both project and global
add_task tools.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/floating-ai-integration-guide.md

@@ -642,7 +642,7 @@ Use these existing patterns from the codebase:
 
 ## Step 5: Add `ai:action` IPC Side-Channel
 
-**Status**: [ ]
+**Status**: [x] (2026-02-28)
 **Prerequisites**: Step 4 completed
 **Modifies**:
 - `src/preload/trpc.ts`